1. Field of the Invention
The present invention relates to a ring switching method that determines a bypass route in a multi-ring network in order to recover the transmission against a failure in the network. Moreover, the present invention relates a node apparatus that performs the ring switching method.
2. Description of the Related Art
Conventionally, when a ring network is constituted by a plurality of SONET (Synchronous Optical Network) devices and a recovery of the transmission over the network against a failure is needed, the BLSR (Bidirectional Line Switched Ring) switch operation is performed in conformity with the GR-1230-CORE of Telcordia Technologies. In this requirement, it is specified that a failure switching request, which is carried by the APS (k1, k2) bytes of the overhead of SONET frame, is notified to a far-end node of the network, and that software switch operation be performed in order to bypass the failure-detected node. Regarding the path with which the transmission is cut off due to the failure, the switching from the working channel to the protection channel is performed through bridge operation at the far-end node. And the switching from the protection channel to the working channel is performed through switch operation at the failure-detected node, in order to avoid the cut-off of the frame transmission.
The above-mentioned conventional method treats the protection channels in a fixed manner when switching the working channel to the protection channel. The bypass route constitutes a closed-loop route within the same ring. In a case of the two-fiber BLSR method, the working channels that amount to 50% of the entire resources and the protection channels that amount to 50% of the entire resources are needed.
An important reason why the BLSR method is demanded for the SONET network is that a bypass route avoiding the transmission through the failure-detected node is taken at the time of detection of a failure, and that the BLSR method is capable of making effective use of the protection channels as the PCA (protective connecting arrangement) circuits during a normal operation, which is different than the UPSR (unidirectional path switched ring) method.
However, the above-mentioned BLSR method treats the protection channels in a fixed manner, and the ratio of the protection channels to the working channels is relatively large. The bypass route that is determined based on the BLSR method constitutes part of a closed-loop path within the same ring. Even when another bypass route passing through different rings exists in the network, the conventional method does not serve to effectively use the protection channels of such another bypass route. It is difficult for the conventional method to avoid the cut-off of the transmission when a multiple failure occurs in the ring to which the node apparatus itself pertains.
Moreover, regardless of how many working channels on a failure-detected line are currently used, all the PCA circuits of the ring network in an opposite direction will be deactivated after the detection of the failure. The conventional method has a problem in that, even when only one working channel is used on the line where the failure is detected, all the PCA circuits of the ring in the opposite direction will be deactivated due to the failure.